drm/i915: Move vbt read from firmware to intel_bios.c

VBT read from firmware is currently nested within opregion vbt read.
Extract it and place it together with other vbt read mechanisms and
dis-associate vbt-firmware from opregion structure.

v2: Return NULL in failure cases and use a null check in
    intel_bios_init(Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240228213235.2495611-4-radhakrishna.sripada@intel.com